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Can Exercise Cause A.L.S.?

Ever since the great Yankees first baseman Lou Gehrig -- pictured here scoring first run of the 1938 World Series -- died of A.L.S. in 1941, many Americans have vaguely connected A.L.S. with athletes and sports.Credit Associated Press

Amyotrophic lateral sclerosis has been all over the news lately because of the ubiquitous A.L.S. ice bucket challenge. That attention has also reinvigorated a long-simmering scientific debate about whether participating in contact sports or even vigorous exercise might somehow contribute to the development of the fatal neurodegenerative disease, an issue that two important new studies attempt to answer.

Ever since the great Yankees first baseman Lou Gehrig died of A.L.S. in 1941 at age 37, many Americans have vaguely connected A.L.S. with athletes and sports. In Europe, the possible linkage has been more overtly discussed. In the past decade, several widely publicized studies indicated that professional Italian soccer players were disproportionately prone to A.L.S., with about a sixfold higher incidence than would have been expected numerically. Players were often diagnosed while in their 30s; the normal onset is after 60.

These findings prompted some small, follow-up epidemiological studies of A.L.S. patients in Europe. To the surprise and likely consternation of the researchers, they found weak but measurable associations between playing contact sports and a heightened risk for A.L.S. The data even showed links between being physically active — meaning exercising regularly — and contracting the disease, raising concerns among scientists that exercise might somehow be inducing A.L.S. in susceptible people, perhaps by affecting brain neurons or increasing bodily stress.

But these studies were extremely small and had methodological problems. So to better determine what role sports and exercise might play in the risk for A.L.S., researchers from across Europe recently combined their efforts into two major new studies.

The more impressive of these, which was published in May in Annals of Neurology, involved almost two dozen researchers from five nations, who developed a deceptively simple but scientifically rigorous research approach. They asked 652 A.L.S. patients if they’d be willing to talk about their lives and activities and did the same with 1,166 people of matching ages, genders and nationalities. They conducted extensive in-person interviews with each volunteer, asking them how active they had been in professional or amateur sports, at their jobs and during leisure time. They also asked about past histories of injuries and accidents, including concussions and other head trauma but also other injuries.

They then compared answers from the people with A.L.S. to those of healthier people.

The results should reassure those of us who exercise. The numbers showed that physical activity — whether at work, in sports or during exercise — did not increase people’s risk of developing A.L.S. Instead, exercise actually appeared to offer some protection against the disease. Even pro athletes showed no heightened risk, although they represented such a tiny subset of the patients with A.L.S. that firm conclusions cannot be drawn, the researchers say.

One aspect of people’s lives did significantly increase their risk of developing A.L.S.: a history of multiple hits to the head. Men and women who had sustained at least two concussions or other serious head injuries were much more likely than other people, including never-concussed athletes, to develop A.L.S.

These results coincide closely with those of the other new study, a review article published in July in the European Journal of Epidemiology, which gathered data from 50 years’ worth of epidemiological studies related to A.L.S. risk (including the other new study) and teased out the effects of physical activity. Most of the studies were limited in scope, but they amplified one another’s validity when combined, the researchers thought.

And their main finding was that “in the general population, physical activity is not a risk factor for A.L.S.,” said Dr. Benoit Marin, a neuroepidemiologist at the French Institute of Health and Medical Research in Paris who oversaw the new review.

But as Dr. Marin also pointed out, the studies involved were all associational, meaning that they cannot establish cause and effect. Exercise and a reduced risk for A.L.S. might be linked to other lifestyle factors, such as a healthy diet, and not to each other.

The new studies also cannot dispel the lingering and troubling questions about the effects of head injuries from contact sports.

“I would not consider this issue settled,” said Ettore Beghi, a neuroscientist at the Mario Negri Institute for Pharmacological Research in Milan and senior author of the study published in May in Annals of Neurology.

In the United States, a few researchers have begun to look at football and A.L.S. risk, a plausible research concern, Dr. Beghi said, given evidence that head trauma sustained playing football might contribute to neurodegenerative diseases. But to date, the football data has been inconclusive.

For now, he and other scientists are continuing to study Italian soccer players, as well as athletes in other sports, including rugby, which, for some reason, confers no increased risk of A.L.S., although it involves considerable contact. Such research may ultimately “shed some light on the underlying mechanisms of the disease, which are still poorly understood,” Dr. Beghi said.

The greatest obstacle to advancing the research, he added, is “the lack of funding,” a situation that could be ameliorated, somewhat, with all of that ice dousing.

A version of this article appears in print on 09/09/2014, on page D4 of the NewYork edition with the headline: Exercise Canu2019t Give You A.L.S..